Conventionally this type of positioner is designed so as to operate with an electric current between 4 and 20 mA (a DC electric signal) sent through a pair of electric wires from a higher-level system. For example, if a current of 4 mA is sent from the higher-level system, the opening of the regulator valve is set to 0%, and if a current of 20 mA is sent, then the opening of the regulator valve is set to 100%.
In this case, the supplied electric current from the higher-level system varies in the range of 4 mA (the lower limit electric current value) through 20 mA (the higher limit electric current value), and thus the internal circuitry within the positioner produces an operating power supply itself from an electric current of no more than the 4 mA that can always be secured as an electric current value that is supplied from the higher-level system. (See, For Example, Japanese Unexamined Patent Application Publication 2004-151941 (“JP '941”).)
The opening setting value for the regulator valve is inputted into the positioner by the higher-level system. Moreover, the actual opening value for the regulator valve is obtained through the opening sensor. Consequently, the positioner is able to perform regulator valve fault diagnostics, self-diagnostics, and the like, through performing calculations on the relationship between the opening setting value and the actual opening value for the regulator valve. The provision of such fault diagnostic functions in the positioner makes it possible to increase the functionality of the system at a low cost, through eliminating the need for providing a separate fault diagnosing device. (See, for example, JP '941.)
For reasons such as these, in recent years there have been proposals for positioners that have, in addition to their actual functions of controlling the degree of opening of the regulator valves, also opening degree transmitting functions, regulator valve fault diagnostics, and functions for sending, to the higher-level system, the results of fault self-diagnostics, and the like. FIG. 8 shows the structure of the critical components of a positioner that has a communication function for the higher-level system. (See, for example, Japanese Unexamined Patent Application Publication H11-304033 (Japanese Patent Application Number 3596293).)
In FIG. 8, the input terminals T1 and T2 input DC electric current signals that are between 4 and 20 mA. A zener diode ZD1 is connected through a resistor RA to the input terminals T1 and T2, to produce a power supply voltage for use by the internal circuitry such as the modem 4 and the CPU 6, and the like. A capacitor CA is inserted between the transmitting circuit 1 and receiving circuit 2 and the input terminal T2, thus providing DC insulation between the power supply voltage V1 and the digital communication signals. A capacitor CB is a decoupling circuit for the power supply voltage V1, to prevent transfer or feedback of energy between the power supply voltage V1 and the ground GND.
A transmitting circuit 1 sends, through digital communications, a response signal to the higher-level system. A receiving circuit 2 receives a request signal, from the higher-level system, requesting a transmission. Here the higher-level system is connected to the input terminals T1 and T2 through two transmission lines (a pair of electric wires). Moreover, the impedance due to the resistor RA is used effectively in the digital communications in the transmitting and receiving circuits 1 and 2 in order to maintain a communication amplitude above a given voltage level. An electric current detecting circuit 3 is for detecting the value of the electric current signal that is inputted into the input terminals T1 and T2, and sends the detected signal to an A/D converting device 7.
A modem 4 is for performing modulation and demodulation of the digital signals of the transmitting and receiving circuits 1 and 2, and exchanges the contents of those signals with a CPU 6. The CPU 6 performs the digital communications and the positional control of a regulator valve 14, and has a communication processing program, for request signals, response signals, and the like, and a controlling program, such as PID, control, or the like, stored in a memory 5. Because the control output of the CPU 6 is a digital signal, it is converted into an analog signal by the D/A converting device 8.
A driving circuit 9 amplifies, and adjusts the impedance of, the analog signal that is sent from the D/A converting device 8, and sends the result to an electropneumatic converting module 11. A sensor interface circuit 10 processes a signal of a position sensor 13, and sends it to the A/D converting device 7. The A/D converting device 7 digitizes the inputted electric current signal from the electric current detecting circuit 3 and the position signal for the regulator valve, sent from the sensor interface circuit 10, and sends the result to the CPU 6.
The electropneumatic converting module 11 is for converting the inputted driving current into a pneumatic signal, and controls the pneumatic pressure of a nozzle through a torque motor. A control relay 12 is for amplifying the pneumatic signal, where the opening and closing of the regulator valve 14 is driven by the amplified pneumatic signal. The opening/closing control of the regulator valve 14 is performed through the position signal of the position sensor 13 being sent to the CPU 6 through the sensor interface circuit 10 and the A/D converting device 7, through the CPU 6 performing controlling calculations, and through the control output being sent to the driving circuit 9 through the D/A converting circuit 8. As a result, the regulator valve 14 is driven to control the degree of opening to the target value through the following path: driving circuit 9→electropneumatic converting module 11→control relay 12→regulator valve 14.
In the positioner 100, an AC electric current signal is superimposed on the DC electric current signal that is between 4 and 20 mA, to enable communication between the system on the higher-level side and the positioner 100. As the content of this communication there are exchanged control parameters for the control calculations pertaining to the regulator valve 14, amounts of adjustment of the zero/span point, the signal outputs of the position sensor, and self-diagnostic results, as the content of the communications. The communication data is read in through the following path: the receiving circuit 2→the modem 4→the CPU 6; and the transmission of the communication data is through the following path: the CPU 6→the modem 4→the transmitting circuit 1. The DC electric current signal inputted into the positioner 100 is recognized through the following path: the electric current detecting circuit 3→the A/D converting device 7→the CPU 6.
In the positioner 100, in order to perform the digital communications, the resistor RA must be above about 250 ohms, so the voltage drop will be more than 5 V with an inputted electric current of 20 mA, causing the voltage V1 that is produced by the zener diode ZD1 to become smaller. Given this, a variable impedance circuit Z1, as illustrated in FIG. 9, is used as an active load instead of the resistor RA.
In the variable impedance circuit Z1, the transistor Q2 has the collector connected to a line L1, and the emitter connected to a line L2 through a resistor R7. The transistor Q1 has its collector connected to the line L1 and connected through a resistor R2 to the base thereof, and the emitter is connected to the base of the transistor Q2, and also connected to the line L2 through a resistor R4. The base of the transistor Q1 is connected to the line L2 through a parallel circuit of the resistor R3 with a capacitor C1 and a resistor R5. Note that the line L1 is a line connected to the zener diode ZD1, and the line L2 is a line connected to the terminal T2.
FIG. 10 is an impedance characteristic diagram for the variable impedance circuit Z1. As can be understood from the impedance characteristic diagram, the variable impedance circuit Z1 has characteristics wherein the impedance (|Z|) is low in the low-frequency domain and the impedance (|Z|) is high in the high-frequency domain. That is, it has characteristics wherein the impedance is low for a DC electric current signal, and the impedance for an AC electric current signal is higher than the impedance for the DC electric current signal. The use of such a variable impedance circuit Z1 makes it possible to reduce the voltage drop in the variable impedance circuit Z1, to thereby increase the voltage V1 that is produced by the zener diode ZD1.
However, in a positioner that uses this variable impedance circuit Z1, as can be understood also from the impedance characteristic diagram illustrated in FIG. 10, the characteristics are gradual at the transition point from the low frequencies wherein the impedance is low to the high frequencies wherein the impedance is high, and thus there is a problem in that it is susceptible to the effects of low-frequency noise.
The present invention was created to solve such a problem, and the object thereof is to provide a positioner that is robust to the effects of low-frequency noise.